Board Games, Organizing, and Wait, Where Did THAT Come From?

I love my board and card games. There are times I wonder if I like organizing them and making inserts and tuckboxes and the like for them at least as much as actually playing them. I've always just kind of built an insert when the mood struck, but I decided this weekend to go through every box and decide if it's ok as is, needs organization that I can do in a few minutes so I do it, or needs a full insert that will take a couple hours (or more) to plot out and build so I add it to a list. It's been fun. :) Then I hit a snag. When I opened my copy of a small box card game (Coup for anyone familiar with it) I discovered some alt art and promo cards that I don't remember ever owning. It is possible I got them at some point and just forgot, but I'm skeptical of that. I wonder if my copy got swapped with someone else's at my big group meetup. I've sent a message to the main organizer asking if anyone has mentioned missing their promo cards, but now I'm wondering if I should do something with my games to mark them. The ones I have custom inserts in and the like are pretty obviously mine, but maybe I should get a stack of business cards to toss in every box or tape to the inside of the lid or something so I can quickly check. That won't stop pieces from migrating from one copy to another or just getting lost, but then at least I'd know if a game box was mine or not.